| abstract | - Gravel mines, also called Button mines were small U.S. made air-dropped anti-personnel mines. They were used extensively during the Vietnam War as part of the McNamara Line. They were also used as a rapid-deployment area denial expedient, to provide a barrier during combat search and rescue (CSAR) operations between downed pilots or other endangered units and infantry threats. The mines consisted of a small green or brown camouflage fabric pouch filled with Lead(II) azide and 30 grams of coarse ground glass between two sheets of plastic. No fuse was required because the explosive became shock-sensitive after dispersal, i.e. able to be detonated without a fuse on contact. The explosive lumps came in wedge or cubed shapes and their plasticizers evaporated after three to eight minutes exposure to air. To allow them to be handled and dropped from the air, the mines were stored soaked in Freon 113. Once released from their container, the Freon would evaporate in between 3 and 8 minutes, thereby arming the mines. The mines varied in size, from simple warning bomblets (Button mines), whose detonation was to be picked up by air dropped acoustic sensors and relayed to a central control centre, through to larger mines, while not powerful enough to kill a person outright, they were capable of wounding anyone stepping on it. The larger mines were fitted with a two tablet chemical system to gradually render the explosive inert, although the reliability of this mechanism is unknown. The mines were also used by the U.S. during the Battle of Khe Sanh, however a U.S. Air Force history described them as being 'little more than a nuisance', with the Viet Cong clearing the gravel mine fields using Oxen dragging logs and the mines becoming inert after a short time. 37 million gravel mines were produced between 1967 and 1968, though mines were produced into 1970.
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